With the development of more and more sophisticated electronic devices, such as cell phones, small laptop computers, sometimes referred to as “netbooks,” electronic or digital assistants, sometimes referred to as “smart phones,” etc., including those capable of increasing processing speeds, display resolution, device features (such as cameras) and higher frequencies, relatively extreme temperatures can be generated. Indeed, with the desire for smaller devices having more complicated power requirements, and exhibiting other technological advances, such as microprocessors and integrated circuits in electronic and electrical components and systems as well as in other devices such as high power optical devices, thermal management is even more important. Microprocessors, integrated circuits, displays, cameras (especially those with integrated flashes, and other sophisticated electronic components typically operate efficiently only under a certain range of threshold temperatures. The excessive heat generated during operation of these components can not only harm their own performance, but can also degrade the performance and reliability of other components, especially adjacent components, and the overall system and can even cause system failure. The increasingly wide range of environmental conditions, including temperature extremes, in which electronic systems are expected to operate, exacerbates these negative effects.
In addition, the presence of heat-generating components can create hot spots, areas of higher temperature than surrounding areas. This is certainly true in displays, such as plasma display panels or LCDs, where temperature differentials caused by components or even the nature of the image being generated can cause thermal stresses which reduce the desired operating characteristics and lifetime of the device. In other electronic devices, hot spots can have a deleterious effect on surrounding components and can also cause discomfort to the user, such as a hot spot on the bottom of a laptop case where is sits on a users lap, or on the touch points on the keyboard, or the back of a cell phone or smart phone, etc. In these circumstances, heat dissipation may not be needed, since the total heat generated by the device is not extreme, but heat spreading may be needed, where the heat from the hot spot is spread more evenly across the device, to reduce or eliminate a hot spot.
With the increased need for thermal management and dissipation from electronic devices caused by these conditions, thermal management becomes an increasingly important element of the design of electronic products. As noted, both performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment. What can complicate these issues, however, is the fact that different devices have different needs. For instance, some devices may need a heat spreader having an in-plane thermal conductivity of 600 W/m*K, while others many need a heat spreader having an in-plane thermal conductivity of 900 W/m*K, etc. (for the purposes of the present disclosure, in-plane thermal conductivity is determined by an Angstrom test method or by other known techniques. In general most techniques have an error measurement of no more than plus or minus ten (±10%) percent). In addition, some devices, especially large display devices such as plasma display or LCD televisions, can generate so much heat that a thin (i.e., less than 50 microns in thickness) heat spreader is “overwhelmed,” such that heat passes through the thickness of the spreader, regardless of its in-plane thermal conductivity.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to commercially as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal conductivity due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from high compression, making it especially useful in heat spreading applications. Sheet material thus produced has excellent flexibility, good strength and a high degree of orientation.
The expanded graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon compression of the sheet material to increase orientation. In compressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the in-plane directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal and electrical properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.
Pyrolytic graphite is a type of graphite which exhibits anisotropic properties. A factor in its anisotropic nature is the continuity of covalent bonding within the plane of the graphene layers. The predominant bonding between the graphene layers is more a function of Van Der Waals interactions than covalent bonding. A difference between the pyrolytic graphite and the natural graphite in this regard is the crystallite size in the pyrolytic graphite is much larger than in the natural graphite. This allows heat to be more efficiently conveyed in the X-Y plane in pyrolytic graphite. Further larger crystallite size means lower contact resistance in the pyrolytic vs. the natural graphite. Generally it is produced by heating a hydrocarbon nearly to its decomposition temperature, and permitting the graphite to crystallize (pyrolysis). One method is to heat synthetic fibers in a vacuum. Another method is to place seeds or a plate in the very hot gas to collect the graphite coating. Pyrolytic graphite sheets usually have a single cleavage plane, similar to mica, because the graphene sheets crystallize in a planar order, as opposed to graphite, which forms microscopic randomly-oriented zones. Because of this, pyrolytic carbon exhibits several unusual anisotropic properties; for the purposes of the present disclosure, however, it exhibits thermal anisotropy, as does expanded graphite sheets. Indeed, pyrolytic graphite sheets are, in general, more thermally conductive along the cleavage plane than expanded graphite, making it one of the best planar thermal conductors available.
Graphitized polyimide is used herein to refer to graphite films with high crystallinity and which can be created by the solid-state carbonization of an aromatic polyimide film followed by a high temperature heat treatment. Graphitized polyimide films also exhibit significant thermal anisotropy.